CVE-2026-44429: Github.com/modelcontextprotocol/registry Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-rqv2-m695-f8j4",
  "modified" : "2026-05-15T23:46:13Z",
  "published" : "2026-05-08T17:18:32Z",
  "aliases" : [ "CVE-2026-44429" ],
  "summary" : "MCP Registry vulnerable to stored XSS in catalogue UI via attribute-quote breakout in publisher-controlled `websiteUrl`",
  "details" : "## Summary\n\nThe public catalogue UI served at `GET /` (file `internal/api/handlers/v0/ui_index.html`) is vulnerable to stored cross-site scripting via the `server.websiteUrl` field of any published `server.json`. Server-side validation in `internal/validators/validators.go` (`validateWebsiteURL`) only checks that the URL parses, is absolute, and uses the `https` scheme; it does not reject quote characters. Client-side, the value is interpolated into a double-quoted `href` attribute via `innerHTML`, using a homegrown `escapeHtml` helper that performs the standard `textContent` → `innerHTML` round-trip. Per the HTML serialisation algorithm, that round-trip encodes only `&`, `<`, `>` and U+00A0 inside text nodes — it does **not** encode `\"` or `'`. A literal `\"` in `websiteUrl` therefore breaks out of the `href` attribute, allowing arbitrary `on*` event handlers to be appended to the same `<a>` element. The Content-Security-Policy on `/` is `script-src 'self' 'unsafe-inline' https://cdn.tailwindcss.com`, so the injected event handlers execute.\n\nAny user able to obtain a publish token (e.g. via `POST /v0/auth/github-at` with their own GitHub account, or `POST /v0/auth/none` on a deployment that has anonymous auth enabled) can plant a poisoned record visible to every visitor of the registry homepage.\n\n## Affected component\n\n- Validator: `internal/validators/validators.go` — `validateWebsiteURL` (lines 153–199)\n- Sink: `internal/api/handlers/v0/ui_index.html` — `toggleDetails(card, item)` at line 432, the `href` attribute built around `escapeHtml(server.websiteUrl)`\n- Helper: `escapeHtml` defined at `internal/api/handlers/v0/ui_index.html` lines 494–498\n\n## Proof of concept\n\n1. Obtain a Registry JWT for any namespace you control (a GitHub OAuth exchange against a throwaway account suffices):\n\n   ```bash\n   TOKEN=$(curl -sS -X POST https://registry.modelcontextprotocol.io/v0/auth/github-at \\\n        -H 'Content-Type: application/json' \\\n        -d '{\"github_token\":\"<gh-pat>\"}' | jq -r .registry_token)\n   ```\n\n2. Publish a server with a poisoned `websiteUrl`. The literal `\"` is preserved end-to-end:\n\n   ```bash\n   curl -sS -X POST https://registry.modelcontextprotocol.io/v0/publish \\\n     -H \"Authorization: Bearer $TOKEN\" \\\n     -H 'Content-Type: application/json' \\\n     --data-binary @- <<'EOF'\n   {\n     \"$schema\": \"https://static.modelcontextprotocol.io/schemas/2025-09-29/server.schema.json\",\n     \"name\":  \"io.github.<your-account>/xss-poc\",\n     \"version\": \"0.0.1\",\n     \"description\": \"hover the website link\",\n     \"websiteUrl\": \"https://example.com/\\\"onmouseover=alert(document.domain)//\"\n   }\n   EOF\n   ```\n\n3. Visit `https://registry.modelcontextprotocol.io/`, search for `xss-poc`, click the card to expand it, then hover the **Website** link in the details panel. The injected `onmouseover` fires and `alert(document.domain)` runs on the `registry.modelcontextprotocol.io` origin.\n\n## Why server-side validation does not catch this\n\nGo's `net/url.Parse` accepts literal `\"` in the path component:\n\n```\ninput=\"https://example.com/\\\"onmouseover=alert(1)//\"  IsAbs=true  Scheme=\"https\"  Path=\"/\\\"onmouseover=alert(1)//\"\n```\n\nNeither the Huma `format:\"uri\"` annotation nor `validateWebsiteURL`'s scheme/`IsAbs` triplet rejects this string. The architecture's existing protection — `repository.url` is regex-locked to `^https?://(www\\.)?github\\.com/[\\w.-]+/[\\w.-]+/?$` and therefore cannot contain quotes — does not extend to `websiteUrl`, which has no allowlist.\n\n## Why client-side `escapeHtml` does not catch this\n\n```js\nfunction escapeHtml(text) {\n    const div = document.createElement('div');\n    div.textContent = text;\n    return div.innerHTML;\n}\n```\n\nPer the HTML5 spec (§13.3 Serialising HTML fragments), the only characters encoded inside the text content of an element are `&`, `<`, `>`, and U+00A0. `\"` and `'` are **not** encoded because in a text-content context they are not special. The helper is therefore safe in element-text contexts (where it is correctly used for `name`, `version`, `description`, etc.) but unsafe inside an attribute value, which is precisely where it is invoked for `href` on lines 432 and 426.\n\n## Impact\n\n- Stored XSS on the official MCP Registry homepage. The malicious entry sits in the public catalogue alongside legitimate ones; any user expanding the entry triggers the payload.\n- Because the page is served on the official `registry.modelcontextprotocol.io` origin, the injected script can:\n  - Read and overwrite `localStorage` (`baseUrl`, `customUrl`), pinning the user's subsequent reads to an attacker-controlled \"Custom\" base URL.\n  - Issue any same-origin or cross-origin XHR (`connect-src *` is granted).\n  - Phish for Registry JWTs by injecting fake auth flows on the trusted origin.\n- The CSP `script-src 'self' 'unsafe-inline' https://cdn.tailwindcss.com` does not block this because `'unsafe-inline'` permits inline event-handler attributes.\n\n## Suggested remediation (any one suffices)\n\n1. Replace the homegrown `escapeHtml` with an attribute-safe encoder that also escapes `\"`, `'`, backtick, and `=` — the OWASP HTML attribute-encoding rule.\n2. Avoid building the `href` via string templates. Use `setAttribute('href', value)` instead — `setAttribute` is not subject to HTML tokenisation, so no breakout is possible.\n3. Tighten `validateWebsiteURL` to reject any URL whose raw bytes contain `\"`, `'`, `<`, `>`, ` `, `\\t`, or `\\n`, or — conservatively — store the canonical re-serialised form (`parsedURL.String()` percent-encodes such characters in the path).\n4. Drop `'unsafe-inline'` from `script-src` after auditing the inline scripts on the page.\n\nApproach (3) is the smallest server-side change and immediately neutralises the exploit for any new publishes; approaches (1) or (2) close the class of bug at the sink so future fields with similar patterns are safe by default.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:N/SI:L/SA:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/modelcontextprotocol/registry"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.7.7"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/registry/security/advisories/GHSA-rqv2-m695-f8j4"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-44429"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/registry/pull/1249"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/registry/commit/78b7bbde07948049b916d76b4769faee461ff930"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/modelcontextprotocol/registry"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/registry/releases/tag/v1.7.7"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-116", "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-08T17:18:32Z",
    "nvd_published_at" : "2026-05-14T21:16:46Z"
  }
}